This invention relates to diapers, catamenial products, and related disposable absorbent products and to the nonwoven fabrics from which these products are constructed.
Diapers, catamenial products, adult incontinence garments, and related disposable absorbent products typically comprise a liquid pervious body side liner, which is a topsheet, a liquid impervious backing sheet, and an absorbent material disposed between the topsheet and the back sheet for storing liquid. The absorbent material typically is a pulp having particles of superabsorbent powders distributed in the pulp. The topsheet separates the wearer""s skin from the absorbent material and from the liquid that is stored in the absorbent material.
It is desirable that the topsheet have rapid strikethrough so that urine, menses, water associated with feces, or other exudates of the body are rapidly brought into contact with the absorbent material and out of contact with the wearer""s skin. It is also desirable that the topsheet have low rewet so that these liquids do not migrate from the absorbent layer through the topsheet and back into contact with the skin.
The topsheet is normally a nonwoven fabric prepared from relatively coarse fibers. The structure of the nonwoven fabric is controlled to provide the balance of properties desired in the topsheet of rapid strikethrough and low rewet. Typically, the nonwoven is a lightweight spunbond coverstock made from continuous filaments, usually monocomponent continuous filaments that are of sufficient denier to substantially preclude wicking of liquids stored in the absorbent core and consequent rewet. The synthetic polymeric filaments from which the topsheet is made normally are hydrophobic. Surfactants sometimes are added to the fabric to promote fluid transport for rapid strikethrough.
Diapers in particular are sometimes subjected to rapid large or repeated insults of urine and feces that sometimes exceed the strikethrough capacity of the topsheet and may exceed the absorbent capacity of the absorbent material. Modern diapers are normally designed with barrier leg cuffs that are at least somewhat impervious to liquids. The leg cuffs act as containment flaps and hold liquid in contact with the topsheet to attempt to substantially preclude leakage for a time sufficient for the liquid to strike through the top sheet and to be stored in the absorbent core.
Containment flaps may be included in the design of other absorbent garments and related products for the containment of liquids and to limit leakage. Containment flaps, including barrier leg cuffs, are normally constructed from nonwoven webs, although these barrier webs typically have different properties from coverstock intended for use as topsheet. For example, it is desirable for the wearer""s comfort for the containment flaps to be liquid impervious for the containment function and to be air pervious to provide for breathability in the fabric to promote dryness and so the that the garment does not cause the wearer to feel uncomfortably hot.
An example of a composite nonwoven fabric said to be suitable for use as a barrier cuff or other containment flap is described in U.S. Pat. No. 5,492,751. This patent describes a trilaminate nonwoven fabric made from a layer of fine meltblown discontinuous fibers sandwiched between two layers of spunbond fabric.
The assembly of a diaper is somewhat problematic. The containment flaps, top sheet, absorbent core, and backsheet all have to be assembled into a single garment having distinct regions with different functions, which means that precision is needed in locating and securing the components in place. The means used to adhere the various components, which typically is hot melt adhesive, cannot be allowed to substantially detract from the functions of the different regions of the garment.
Problems have arisen in the attachment of barrier leg cuff to the diaper components. Sometimes the seal between the leg cuff and the diaper components to which the leg cuff is attached is defective, which tends to promote leakage around the leg cuff. If the topsheet has been treated to promote strikethrough with a surfactant that extends beyond the leg cuff, then liquid tends to migrate under the leg cuff and into a region sometimes referred to as the ear, where leakage can occur.
The cost to produce diapers and other disposable absorbent articles is directly related to the cost of the materials that are used to make the product and the cost of assembly of the components. The cost to produce a nonwoven fabric is directly related to the amount of polymer from which the web is made, which is to say that the cost is directly proportional to the basis weight. There is an economic incentive in coverstock production for topsheet to obtain the desired balance of strikethrough and rewet characteristics at ever reduced basis weights.
However, basis weight cannot be reduced below the point where specified properties of the fabric are lost. For example, if the basis weight is too low, then the topsheet fabric no longer has sufficient transport capability to provide the desired rapid strikethrough properties and no longer has sufficient barrier capability to provide the desired low rewet properties. Too low basis weight tends to result in migration of superabsorbent particles across the topsheet into contact with the wearer""s skin. Too low basis weight also tends to result in glue bleed through in which adhesive penetrates the topsheet and precludes construction of a satisfactory assembly.
Efforts have been made to prepare topsheet that has regions of liquid permeability and liquid impermeability so that, for example, a diaper or catamenial product has a topsheet region providing for strikethrough, but is liquid impermeable outside this region for liquid containment in the absence of leakage. For example, U.S. Pat. No. 5,431,643 describes a topsheet for use in feminine napkins wherein the topsheet has areas of liquid permeability and areas of liquid impermeability. The impervious areas are formed after the fabric has been produced by application of an impervious coating to selected regions of the fabric by a silk screen printing operation. However, the impervious coating renders the fabric with little or no breathability in the coated areas.
U.S. Pat. No. 5,456,971 describes a compound nonwoven web intended for use as a topsheet in diapers and the like in which the web is formed with separate regions within a single layer for liquid permeability and liquid impermeability. Fine and coarse filaments, which can be meltblown and spunbond filaments, respectively, are admixed with no discrete phase boundary so as to form the liquid impermeable regions. The compound web differs in this respect from a composite fabric structure made of more than one layer. The compound web structure allows fine fibers to contact the skin of the wearer, which could promote wicking of liquid from the absorbent material and consequent undesirable rewet performance.
Design and assembly of diapers and related products would be greatly simplified and the costs of production would be reduced if a single fabric could be satisfactorily used at relatively low basis weight for both containment and liquid permeable topsheet that did not have the above noted drawbacks and disadvantages. It would be desirable to produce a fabric that had separate regions for strikethrough and barrier, could be used in assemblies that were not prone to leakage, and that had a balance of properties so that the strikethrough region had acceptable low rewet characteristics and the barrier region had acceptable breathability and low rewet.
The invention comprises a composite nonwoven structure in which the physical properties of the composite nonwoven structure are manipulated to cause the structure to have good strikethrough and rewet characteristics in selected regions of the fabric while maintaining barrier properties in other regions of the fabric without sacrificing breathability. The composite nonwoven structure is useful as coverstock for disposable absorbent articles and can be used to form a one piece topsheet and barrier fabric for use in diapers. Barrier leg cuffs and other containment structures or barrier zones can be formed in the composite nonwoven fabric adjacent the strikethrough portions of the fabric for simplified garment construction and in the absence of leakage.
The barrier regions can be formed by judicious placement of fine fibers having barrier characteristics within the composite. Alternatively, the entire composite can be prepared as a barrier fabric and the structure can be treated with a suitable surfactant to permit fluid flow in selected regions. Capillary movement within the fluid flow region can be achieved by choice of filament diameters and by controlling the size of the pores formed between adjacent filaments.
In one embodiment of the invention, the composite nonwoven structure is a spunbond-meltblown-spunbond trilaminate structure in which a layer of substantially discontinuous meltblown polymeric fiber is sandwiched between two layers of spunbond continuous filaments. This structure has superior fluid barrier properties at equal basis weights in comparison to fabrics made only of spunbonded filaments. While not wishing to be bound by theory, it is believed that the finer meltblown fibers bridge the voids between the much larger spunbonded filaments to produce improved fluid barrier properties by reducing effective pore size. A surfactant is selectively applied to the fabric to create the fluid flow regions and to promote strikethrough.
In another embodiment, a composite structure is produced in which substantially discontinuous meltblown polymeric fiber is selectively placed between layers of spunbond. A composite fabric is produced that has barrier properties in selected regions in which meltblown is present. For example, the meltblown fibers can be deposited in longitudinal stripes that extend the length of the spunbond layer in the machine direction. The regions without meltblown fibers do not have the barrier properties typically associated with a spunbond-meltblown-spunbond trilaminate structure. A surfactant can be selectively applied to the fabric in the regions in the absence of meltblown to create the fluid flow regions and to promote strikethrough.
In additional embodiments, although not necessarily with equivalent results, the composite nonwoven fabric can be produced with a single layer of spunbond and a layer of meltblown deposited thereon either as a continuous layer or in longitudinal stripes that extend the length of the spunbond layer in the machine direction. In this embodiment, the fabric is oriented in disposable absorbent articles so that the spunbond layer faces the skin of the wearer and the meltblown layer faces the absorbent material so as to reduce the likelihood of rewet.
It should be recognized that the barrier stripes can alternated with coarse spunbond stripes between the barrier stripes, if desired, in any of the composites of the invention. It should be recognized that either the barrier stripes or the continuous layer of barrier can comprise fine continuous spunbond filaments that have barrier properties. Typically, these filaments are of less than one denier, whereas more coarse spunbond filaments are used in the layers facing the skin to substantially reduce wicking and rewet. Mixtures of meltblown and spunbond having sufficient barrier properties can be used in compound web regions within the composite structure.
In another embodiment, the invention provides a topsheet suitable for a diaper or other disposable absorbent article that can be produced with a central region that has the characteristics of rapid strikethrough and low rewet for distribution of body fluids to the absorbent layer. A peripheral region outside the central region can retain its barrier properties sufficient to be useful as a containment flap, leg cuff, or barrier zone so as to substantially preclude leakage of body fluids.
Thus, the invention provides, among other things, a nonwoven fabric that is suitable for use as a one-piece topsheet and barrier fabric in which a single fabric is used for both leg cuff and topsheet. The invention substantially eliminates the need for separate containment flaps applied to a topsheet and also provides a breathable fabric.